1. Field of the Invention
The present invention relates to ultrasound imaging and, more particularly, to a color Doppler ultrasound imaging system.
2. Description of the Related Art
Ultrasound imaging devices are known which support simultaneous B mode, color mode, and Doppler mode imaging (referred to as "BcD mode" imaging). In such systems, the B mode ultrasound produces a two-dimensional tomographic image of the object being scanned, and the color and Doppler modes are used to generate an image of blood flow or tissue fluid movement information that is spatially coordinated with and superimposed upon the B mode two-dimensional tomographic image.
Doppler ultrasound is based on the Doppler effect, which is a change in frequency caused by the relative motion of a wave source, a receiver and a reflector. As applied to medical applications, an ultrasound transducer embodying the source and receiver is stationary while blood or tissue fluid is the moving reflector. The change in frequency detected is the difference between the transmitted ultrasound signal frequency and the reflected ultrasound signal frequency. The change is a function of the transmitted signal frequency, the propagation speed of the transmitted signal through the patient's anatomy, the speed of flow in the range gate and the angle of incidence between the ultrasound signal and the direction of blood flow.
In B mode imaging, ultrasonic pulses are radiated to a subject and the radiation direction is scanned in order to obtain the tomographic image. In Doppler mode, ultrasonic pulses are transmitted repeatedly in respective directions in which ultrasonic waves are radiated, and a Doppler shift frequency is detected based on the phase variation of the reflected waves. More particularly, to obtain the blood flow data, an ultrasound probe or transducer is driven to repeatedly radiate ultrasonic waves in a particular direction for a number of times, and the received signal is detected by an orthogonal phase detecting circuit, thereby obtaining a Doppler shift signal on the basis of blood or fluid flow.
The Doppler shift signal is frequency analyzed by a frequency analyzing circuit to find an average value of the Doppler shift, an average power of the Doppler shift, etc. A blood flow velocity color flow mapping image is obtained by a Fast Fourier Transform (FFT) circuit and the blood flow velocity color flow mapping image and a B mode image are provided to a digital scan converter. The images are then read out of the scan converter and the two-dimensional blood flow velocity color flow mapping image is superimposed on the B mode display on a monitor.
In BcD mode imaging, color mode transmission vectors are typically interleaved with Doppler vectors or pulses, rather than transmitted successively. The interleaving is defined by an interleave factor, which refers to the number of color vectors sent between the Doppler vectors. Interleaving color mode vectors with the Doppler vectors is necessary in order to increase frame rate (i.e., the rate of image display). However, interleaving the color mode vectors with the Doppler vectors can result in a residue effect, which result from reflections of the interleaved waves that have not yet died out before the Doppler reading occurs. If there is any residue signal due to the color, the Doppler signal will be corrupted. This can result in inaccurate data and what is referred to as broken tone artifacts.
Accordingly, there is a need for a BcD mode ultrasound system in which broken tone artifacts due to the interleaving factor of the color mode on the Doppler spectrum is eliminated. There is a still further need for a Doppler ultrasound system in which Doppler signals are optimized.